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United States Patent Application |
20060186429
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Kind Code
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A1
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Chew; Tong Fatt
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August 24, 2006
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Semiconductor light emitting device and method of manufacture
Abstract
A light-emitting diode ("LED") device has an LED chip attached to a
substrate. The terminals of the LED chip are electrically coupled to
leads of the LED device. Elastomeric encapsulant within a receptacle of
the LED device surrounds the LED chip. A second encapsulant is disposed
within an aperture of the receptacle on the elastomeric encapsulant.
Inventors: |
Chew; Tong Fatt; (Penang, MY)
|
Correspondence Address:
|
AVAGO TECHNOLOGIES, LTD.
P.O. BOX 1920
DENVER
CO
80201-1920
US
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Serial No.:
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068645 |
Series Code:
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11
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Filed:
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February 22, 2005 |
Current U.S. Class: |
257/100; 257/E33.059 |
Class at Publication: |
257/100 |
International Class: |
H01L 29/22 20060101 H01L029/22 |
Claims
1. A light-emitting diode ("LED") lamp comprising: a substrate; an LED
chip having a first terminal and a second terminal attached to the
substrate; a first lead electrically coupled to the first terminal of the
LED; a second lead electrically coupled to the second terminal of the
LED; a receptacle having an aperture, the LED chip and at least a portion
of the substrate being inserted into the receptacle through the aperture;
elastomeric encapsulant within the receptacle surrounding at least the
LED chip; and rigid encapsulant disposed within the aperture of the
receptacle on the elastomeric encapsulant.
2. The LED lamp of claim 1 wherein the elastomeric encapsulant comprises
silicone encapsulant.
3. The LED lamp of claim 1 wherein the first lead is electrically coupled
to the first terminal of the LED with a bond wire.
4. The LED lamp of claim 1 wherein the LED chip is a flip-chip LED.
5. The LED lamp of claim 1 wherein the LED chip includes a
wavelength-converting overlay.
6. The LED lamp of claim 5 wherein the wavelength-converting overlay
comprises wavelength-converting particles dispersed in an elastomeric
matrix.
7. The LED lamp of claim 5 wherein the wavelength-converting overlay
comprises phosphor particles dispersed in a silicone matrix.
8. The LED lamp of claim 1 wherein the LED chip emits light having a
wavelength between 200 nm and 570 nm.
9. The LED lamp of claim 1 wherein the second encapsulant comprises a
rigid polymer with a Shore Hardness greater than D70.
10. The LED lamp of claim 1 wherein the second encapsulant comprises a
thermally conductive filler dispersed in a rigid polymer.
11. The LED lamp of claim 1 wherein the second encapsulant has a thermal
conductivity greater than 0.5 W/m.degree. K.
12. The LED lamp of claim 1 wherein the first lead and the second lead are
cut so as to singulate the LED lamp from a leadframe strip.
13. The LED lamp of claim 12 wherein the substrate is a metal substrate
integrated with the second lead and the LED chip is conductively attached
to the metal substrate.
14. The LED lamp of claim 1 wherein the substrate is a printed circuit
board ("PCB") substrate.
15. The LED lamp of claim 1 wherein the first lead is a first backside
terminal of the substrate and the second lead is a second backside
terminal of the substrate, the second encapsulant partially encapsulating
the substrate so as to seal the elastomeric encapsulant within the
receptacle while allowing electrical connections to be made to the first
backside terminal and to the second backside terminal.
16. The LED lamp of claim 1 wherein the substrate is a gull-wing substrate
or a J-lead substrate.
17. The LED lamp of claim 1 further comprising a plurality of LED chips
wherein the receptacle has a plurality of mold cups, at least one LED
chip emitting light into each of the plurality of mold cups, the
elastomeric encapsulant surrounding each of the LED chips.
18. The LED lamp of claim 1 wherein the elastomeric encapsulant has a
first index of refraction and forms a primary lens and the receptacle has
a second index of refraction and forms a secondary lens, the first index
of refraction being selected to be greater than the second index of
refraction.
19. A method of manufacturing a light-emitting diode ("LED") device
comprising: die-attaching an LED chip to a substrate; electrically
connecting the LED chip to leads of the LED device; dispensing uncured
elastomeric encapsulant into a receptacle of the LED device through an
aperture of the receptacle; inserting the LED chip and substrate through
the aperture so as to surround the LED chip with elastomeric encapsulant;
and curing the elastomeric encapsulant.
20. The method of claim 19 further comprising, after the step of inserting
the LED chip and substrate through the aperture, of: dispensing a second
encapsulant into the receptacle over the cured elastomeric encapsulant;
and curing the second encapsulant.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
REFERENCE TO MICROFICHE APPENDIX
[0003] Not applicable.
BACKGROUND OF THE INVENTION
[0004] A conventional light emitting diode ("LED") device uses an epoxy as
encapsulating material. The encapsulation process is frequently
accomplished by injection molding, transfer molding or casting. Cured
epoxy encapsulant has relatively high hardness, which provides resistance
to scratches and abrasion, high rigidity, and high initial light
transmissivity. Conventional encapsulated LED devices come in a variety
of sizes and styles, such as 4 mm Oval LED Lamps, 5 mm Round LED Lamps,
Chip LEDs and plastic leaded chip carriers ("PLCCs").
[0005] However epoxy-based encapsulating materials suffer from thermal and
photo degradation. Degradation is especially acute if the wavelength
emitted by the LED chip is in the near the ultraviolet ("UV") portion of
the spectrum. Epoxy encapsulating material degrades when subjected to
high light flux, particularly if the wavelength of the light is in the
range from 200 nm to 570 nm. Degradation of the encapsulant results in
increased absorption of light in the blue to green wavelengths, causing a
"yellowing" effect on clear epoxy encapsulant and reduced light
transmissivity through the encapsulant, which causes a significant drop
in the light output of the LED device. Typically, an epoxy-based 5 mm LED
lamp device's light output drops by 20% or more after 1000 hours in use,
and by 50% or more after 10,000 hours in use.
[0006] FIG. 1A shows a portion of a strip 100 of semi-finished
conventional LED lamps 102, 104. The LED lamps 102, 104 are attached to a
leadframe 106, which is fabricated as a strip of LED lamps. LED lamps are
singulated from the leadframe 106 by shearing leads 108, 110.
"Singulation" means separating an LED lamp or a group of associated LED
lamps from a leadframe or other substrate, such as a ceramic substrate or
a printed circuit board ("PCB") substrate.
[0007] An LED chip 114 is attached to a first substrate portion 115 that
electrically couples a first terminal (not shown) of the LED chip 114 to
the lead 110. In a particular embodiment, the LED chip is mounted in a
reflector cup of the first substrate portion using conductive epoxy. A
bond wire 112 electrically couples a second terminal (not shown) of the
LED chip 114 to a second substrate portion 117. The bond wire 112, LED
chip 114, first substrate portion 115, second substrate portion 117, and
portions of the leads 108, 110 are encapsulated in hard, rigid
encapsulant 116, such as an epoxy encapsulant. The hard, rigid
encapsulant 116 protects the bond wire from being damaged when an LED
lamp is sheared from the leadframe 106 by securing the first and second
substrate portions so that the do not move relative to each other.
[0008] FIG. 1B shows a conventional singulated LED lamp 102. The leads
108, 110 have been cut from the lead frame (see FIG. 1A, ref. num. 106,
108, 110). One lead 108 has been cut shorter than the other lead 110 to
indicate the electrical polarity of the LED chip 114. Hard, rigid
encapsulant 116 secures one lead relative to the other to prevent avoid
damage to the bond wire 112.
[0009] FIG. 1C shows a conventional singulated LED lamp 102 inserted and
clinched into a PCB 120. In a typical automated assembly process, the
leads 108, 110 of the LED lamp 102 are inserted through holes in the PCB
120 and bent to secure the LED lamp 102 in place for a subsequent
soldering step. The strong, rigid epoxy encapsulant secures the leads
108, 110 from movement relative to each other, which could otherwise
damage the LED chip 114 and/or the bond wire 112.
[0010] FIG. 2 shows a prior-art light source 200. An LED chip 214 is
attached to a PCB substrate 201. Bond wires 212, 213 electrically couple
terminals (not shown) on the LED chip 214 to terminals 208, 210 on the
PCB substrate 201. The terminals 208, 210 are plated through holes that
allow surface mounting of the light source 200 on a surface-mount circuit
substrate. The plated through holes are plugged with a compound 211, such
as solder resist, before encapsulant 216 shaped as a dome is molded over
the LED chip 214 and top of the PCB substrate 201. More information on
such a light source is found in U.S. Pat. No. 6,806,583.
[0011] FIG. 3 is a simplified cut-away isometric view of another prior art
LED device 300. An LED chip 314 is attached to a silicon substrate member
301 (forming what is commonly known as a "chip-on-chip" assembly), which
is attached to a substrate member 303. In a particular embodiment, the
LED chip 314 is a high-power LED chip, that is, an LED chip operating at
or above 500 mW, and the substrate member 303 is metal (e.g a metal
"slug") that provides a heat sink to conduct heat away from the LED chip
314 through the silicon substrate member 301. A lead 308 extends from a
lead support member 309, which is typically a molded polymer. A second
lead is not shown in this view, but essentially extends from the lead
support member opposite the lead 308.
[0012] A pre-molded thermoplastic cover 316 fits over the LED chip 314 to
form a cavity 315 within the LED device 300. The cover 316 is shaped to
form a lens according to the desired light intensity distribution pattern
of the LED device 300. The cover forms a cavity 315 in which the LED chip
314 sits. Subsequently liquid silicone encapsulant is introduced into the
cavity by dispensing or injecting it through an opening in the package to
fill the entire space in the cavity 315 within the package, and the
encapsulating material is then cured. The silicone filling the cavity 315
provides a soft, optically transparent material having a refractive index
greater than 1.3.
[0013] Using silicone materials with high-power LED chips, particularly
those operating in the blue to green portion of the spectrum, is
desirable because silicone is less prone to yellowing than the epoxy used
in LED lamps such as the LED lamp 102 shown in FIG. 1B. However, the
packaging techniques used in the LED device 300 of FIG. 3 are relatively
elaborate, involving many parts and assembly steps.
[0014] Hence, there is a need for LED devices that do not degrade like
conventional LED lamps, yet do not require the number of components and
assembly steps typically used to package high-power LED chips.
BRIEF SUMMARY OF THE INVENTION
[0015] A light-emitting diode ("LED") device has an LED chip attached to a
substrate. The terminals of the LED chip are electrically coupled to
leads of the LED device. Elastomeric encapsulant within a receptacle of
the LED device surrounds the LED chip. A second encapsulant is disposed
within an aperture of the receptacle on the elastomeric encapsulant.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1A shows a portion of a strip of semi-finished conventional
LED lamps.
[0017] FIG. 1B shows a singulated conventional LED lamp.
[0018] FIG. 1C shows a conventional singulated LED lamp inserted and
clinched into a PCB.
[0019] FIG. 2 shows a prior-art light source.
[0020] FIG. 3 is a simplified cut-away isometric view of another prior art
LED device.
[0021] FIG. 4 shows a simplified cross section of an LED device ("lamp")
according to an embodiment of the invention.
[0022] FIGS. 5A-5C show alternative receptacles according to embodiments
of the invention.
[0023] FIG. 6A is an isometric view of a receptacle having a round flange
and a round rim according to an embodiment of the invention.
[0024] FIG. 6B is an isometric exploded view of a receptacle having a
square flange and square rim according to an embodiment of the invention.
[0025] FIGS. 7A-7C show cross sections of portions of LED lamps to
illustrate a manufacturing sequence according to an embodiment of the
invention.
[0026] FIG. 8 is a cross section of an LED lamp according to another
embodiment of the invention.
[0027] FIG. 9 shows a cross section of an LED lamp according to another
embodiment of the invention.
[0028] FIG. 10 shows a cross section of an LED lamp according to another
embodiment of the invention.
[0029] FIG. 11A shows a cross section of an LED lamp according to another
embodiment of the invention.
[0030] FIG. 11B is a plan view of the substrate of FIG. 11A.
[0031] FIGS. 12-14 are isometric views of receptacles configured to
accommodate more than one LED chip.
[0032] FIG. 15 is a flow chart of a method of manufacturing an LED device
according to an embodiment of the invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0033] Silicone elastomers offer desirable characteristics as an
encapsulation material. Silicone elastomers offer high thermal stability,
low photo-degradation, low light loss transmission characteristics, a
wide range of refractive indices, low stress after encapsulation cure,
and low cost. They are non-toxic and are not sensitive to high humidity
high temperature environments. Silicone encapsulants are particularly
desirable for use with light emissions with a wavelength ranging from 200
nm to 570 nm because of the low transmission loss, and more particularly
when used in high-temperature operations (i.e. up to 100.degree. C.).
[0034] However, a cured silicone encapsulant typically has a hardness of
less than Durometer Shore 70A. The silicone polymer may be liquid,
gelatinous or in solid state under various stages of manufacturing.
However the low hardness of silicone polymer has the disadvantages of low
resistance to scratches, abrasion and wear. Furthermore, packages formed
from silicon polymer do not have high rigidity and do not provide good
dimensional stability when subject to mechanical handling, thus reducing
its ease of assembly and limiting the scope of its use in various
preferred LED package outline designs.
[0035] If the epoxy encapsulant 116 of the LED lamp 102 shown in FIG. 1A
were replaced with a silicone encapsulant, the leads 108, 110 and first
and second substrate portions 115, 117 would not be held sufficiently
rigid to avoid damaging the wire bond when the LED lamp is singulated
from the lead frame, or when the leads are bent after insertion through a
PCB (see FIG. 1C). The exterior surface of the LED lamp having the
silicone encapsulant would also be more prone to scratches, wear, and
abrasion that degrade optical performance. Thus, a reliable LED lamp
similar to the prior art device of FIG. 1B is not obtained by using
silicone for the encapsulant due to the low hardness and low rigidity of
typical silicone materials.
[0036] Another problem arises if the epoxy encapsulant is replaced with a
silicone-type encapsulant in an LED lamp using a silicone-phosphor
wavelength-converting layer. The relatively weaker adhesion strength of
the silicone-phosphor coating to the LED chip compared to the adhesion
strength between the silicone-phosphor coating and the epoxy encapsulant
causes the silicone-phosphor layer to de-laminate from the surface of the
LED chip.
[0037] Silicones typically generate less compressive or tensile stresses
on the LED chip in a package after curing compared to harder-cured
polymers. The reduced stress on the LED chip enables a more reliable and
longer-lived LED device. Silicone is known to be especially desirable for
use with LED chips that have a thin, uniform layer of
wavelength-converting material (e.g. phosphor particles and/or quantum
dot particles dispersed in a silicone matrix). Small particles of silica
are optionally added as a diffusant. Such "phosphor layers" are typically
less than about 10% of the thickness of the LED structure, and enable
light emitted from the LED chip to traverse through a uniform path length
resulting in a uniform proportion of light being converted in the
phosphor layer. This results in a uniform color output from the LED
device with respect to the spatial distribution of light. The
silicone-silicone interface between the phosphor layer and silicone
encapsulant is known to be minimally affected by chemical incompatibility
and by mechanical incompatibility during thermal excursion.
[0038] FIG. 4 shows a simplified cross section of an LED device ("lamp")
400 according to an embodiment of the invention. The LED lamp 400 is
round when viewed on end, and has a receptacle 402, which is usually
transparent, but alternatively is diffusive, with high hardness, high
rigidity and high light transmissivity, an elastomeric encapsulant 404
disposed in the receptacle 402. The receptacle 402 may be formed by
injection molding or transfer molding, for example, with the material
used being of substantially transparent material of high rigidity, and
high scratch resistance, for example, but not limited to, materials such
as polycarbonate, cyclic olefin polymers or copolymers, polyamides,
polymethylacrylates ("PMMAs"), liquid crystal polymers ("LCP"), epoxies
and polysulfones. As used herein, "transparent" means clear or tinted. In
some embodiments, the LED chip emits a wavelength between about 200 nm
and about 700 nm.
[0039] An LED chip 408 is mechanically attached to a first substrate
portion 406 in an optional reflector cup 410. A first terminal (not
shown) of the LED chip is electrically coupled to the first substrate
portion 406 with conductive adhesive. A second terminal (not shown) of
the LED chip is electrically coupled to a second substrate portion 407
with a bond wire 416. Elastomeric encapsulant 404 is dispensed into the
receptacle 402, and the LED chip 408, first and second substrate portions
406, 407, and bond wire 416 are inserted into the elastomeric encapsulant
404, which is then cured.
[0040] The receptacle 402 functions as a mold-cup that forms part of the
finished LED lamp 400. The receptacle 402 becomes part of the finished
LED lamp 400, thus avoiding de-lamination problems that can arise when
elastomeric encapsulant is removed from a mold. Suitable elastomeric
encapsulants include silicones, fluorosilicones, perfluoropolymers, and
amphorous fluoroplastics. Suitable encapsulants include encapsulants
described in U.S. Patent Application Publication No. US 2004/0198924 A1,
entitled "Optically clear high temperature resistant silicone polymers of
high refractive index," by Young et al., and are available from NUSIL
TECHNOLOGY of Carpenteria, Calif.
[0041] A first lead 412, which was formerly part of a metal leadframe
strip, extends away from the first substrate portion 406. A second lead
414, which was also formerly part of the metal leadframe strip, extends
away from the second substrate portion 407.
[0042] The receptacle 402 has a substantially non-uniform thickness that
forms a lens for directing light from the LED chip 408. Another lens is
formed by the elastomeric encapsulant 404 according to the inside
curvature of the receptacle 402. The combination of the lens formed by
the receptacle and the lens formed by the elastomeric encapsulant is
particularly desirable to obtain light distribution patterns that are not
achieved with conventional LED encapsulants (e.g. ref. num. 116, FIG.
1A). In a particular embodiment, the elastomeric encapsulant and the
material of the receptacle are selected such that the index of refraction
of the elastomeric encapsulant material is greater than the index of
refraction of the receptacle material to minimize Fresnel light
transmission losses. The higher index of refraction for the elastomeric
encapsulant material provides improved efficiency.
[0043] After the elastomeric encapsulant 404 has been cured, a second
encapsulant 418 is backfilled over the elastomeric encapsulant 404. The
second encapsulant is rigid after curing, and in a particular embodiment
has a Shore Hardness greater than D70, which is desirable to secure the
leads during singulation and during assembly of the LED lamp into a
circuit. The backfilling is done by dispensing uncured second encapsulant
in a liquid or a gel state. After curing, the second encapsulant 418
forms a thermally conductive, hard, rigid layer on top of the elastomeric
encapsulant 404, sealing the LED chip 408 and the elastomeric encapsulant
404 within the receptacle 402. In a particular embodiment, the second
encapsulant 418 includes thermally conductive filler, such as ceramic
(e.g. silicon-aluminum oxide) powder, dispersed in an epoxy matrix. In a
particular embodiment, the second encapsulating material has a thermal
conductivity greater than 0.5 W/m.degree. K.
[0044] The second encapsulant 418 layer provides a stronger, more rigid
base for the leads 412, 414, reducing potential damage to the LED chip
408 and bond wire 416 during singulation, handling, and assembly
operations. The second encapsulant 418 also assists in dissipating heat
generated by the LED chip 408 during operations by conducting heat away
from the substrate 406, both along the leads 412, 414, as well as to the
receptacle 402.
[0045] FIGS. 5A-5C show alternative receptacles according to embodiments
of the invention. FIG. 5A shows a cross section of a receptacle 502
similar to the receptacle 402 of FIG. 4, with the addition of a flange
504. The flange facilitates manufacturability and assembly of an LED lamp
by a user in some applications. FIG. 5B shows a cross section of a
receptacle 506 with a substantially uniform wall thickness, according to
another embodiment of the invention. FIG. 5C shows a cross section of a
receptacle 508 with a substantially uniform wall thickness having a
flange 510. Alternative embodiments use other shapes. In particular
embodiments, the wall thickness of a receptacle is chosen so as to
provide, when filled with elastomeric encapsulant, a desired spatial
distribution of light intensity from an LED device.
[0046] FIG. 6A is an isometric view of a receptacle 600 having a round
flange 602 and a round rim 604 according to an embodiment of the
invention. A rim is useful when an LED lamp is inserted through a PCB to
limit the height of the LED lamp. FIG. 6B is an exploded isometric view
of an LED lamp 606 with a receptacle 608 and a substrate 610. In a
particular embodiment, the substrate 610 is PCB substrate that has
cathode electrical contact pad 613, a center pad 612, which serves as a
heat sink pad in some embodiments and as a second cathode pad in
alternative embodiments, and an anode electrical contact pad 614 on the
back side of the substrate 610. An LED chip (not shown) is mounted in a
reflector cup on the opposite side of the substrate 610 and is
electrically coupled to the contact pads. The receptacle 608 has a square
flange 616 and a square rim 618.
[0047] Alignment holes 620 in the substrate 610 cooperate with alignment
pins 622 on the receptacle 608 to align the substrate, and hence the LED
chip (not shown) to the receptacle. Elastomeric encapsulant (not shown)
is dispensed into a mold cup 624 of the receptacle 608, the substrate 610
is assembled with the receptacle 608, and the elasotmeric encapsulant is
cured. Rigid second encapsulant is optional in this embodiment. The
cathode electrical contact pad(s) and the anode electrical contact pad
are not covered with encapsulant, and are available for surface mounting
the LED lamp on a surface-mount circuit board. Other shapes and styles of
receptacles are used in alternative embodiments. In particular,
alternative embodiments use receptacles configured to accept several
LEDs, such as those shown in FIGS. 12-14.
[0048] FIGS. 7A-7C show cross sections of portions of LED lamps to
illustrate a manufacturing sequence according to an embodiment of the
invention. In FIG. 7A, a receptacle 702 is filled with uncured
elastomeric encapsulant material 704, such as silicone encapsulant
material in a liquid or a gel state. An LED chip 708 has been
die-attached to a substrate 706 that is part of a leadframe strip 710. A
fixture 712 holds the receptacles during a curing step. Alternatively,
empty receptacles are placed into the fixture 712, which are then filled
with elastomeric encapsulant. A rim 714 supports the receptacle 702 in
the fixture 712.
[0049] FIG. 7B shows the substrate 706 and LED chip 708 after it has been
inserted into the fluid elastomeric encapsulant and is later cured. FIG.
7C shows a second encapsulant 716 dispensed over cured elastomeric
encapsulant 704 and around leads 718, 720. The second encapsulant is then
cured. After curing, the second encapsulant 716 is harder and stronger
than the cured elastomeric encapsulant 704. The thickness of the second
encapsulant material is merely exemplary. In a particular embodiment, the
second encapsulant material is thicker than shown, and extends further
toward the surface of the substrate, which enhances heat conduction and
provides more support to the leads and substrate.
[0050] FIG. 8 is a cross section of an LED lamp 800 according to another
embodiment of the invention. An LED chip 808 is a flip-chip LED with a
silicon submount (not separately shown). Flip-chip LEDs are discussed
further in U.S. Pat. Nos. 6,521,914 and 6,646,292. The LED chip 808 has
both an anode bonding pad (not shown) and a cathode bonding pad (not
shown) on the same side of the device. Wire bonds 816, 817 electrically
connect the terminals of the LED chip 808 to the leads 412, 414.
[0051] Second encapsulant 818 is backfilled to substantially surround the
substrate 406 up to near to the LED chip 808 to provide a shorter thermal
path from the flip-chip LED to the secondary encapsulant 818. The thicker
secondary encapsulant also provides a stronger and more rigid base for
the substrate 406 and leads 412, 414. Other embodiments include a number
of LED chips in a single LED package.
[0052] FIG. 9 shows a cross section of an LED lamp 900 according to
another embodiment of the invention. A substrate 906 is a PCB substrate
with pins (leads) 912, 914 configured as leads according to a dual
in-line package ("DIP") standard. This embodiment is especially desirable
for through-hole mounting. Elastomeric encapsulant 904 inside a
receptacle 902 covers an LED chip 908 and associated wire bond. Cured
second encapsulant 918 is harder and more rigid than the elastomeric
encapsulant 904 and provides a secure seal, securely fastens the
substrate 906 within the receptacle 902, and further supports the pins
912, 914.
[0053] FIG. 10 shows a cross section of an LED lamp 1000 according to
another embodiment of the invention. A substrate 1006 is a PCB with
backside terminals 1012, 1014 to form an LED component for surface mount
technology ("SMT") applications. The substrate 1006 is sufficiently thick
to allow positioning the substrate 1006 so that an LED chip 1008 mounted
on one side of the substrate 1006 is encapsulated by elastomeric
encapsulant 1004, while the backside terminals 1012, 1014 are exposed on
the opposite side of the substrate 1006. LEDs suitable for SMT
applications are further described in U.S. Pat. No. 6,806,583, entitled
Light source, by Koay et al. The substrate 1006 is partially encapsulated
by elastomeric encapsulant 1004 and partially encapsulated by a secondary
encapsulant layer 1018, which has higher hardness, higher rigidity, and
is more thermally conductive than the elastomeric encapsulant.
Alternative embodiments use a ceramic substrate, such as alumina or
aluminum nitride, with a reflector cup plated with a reflective material
such as silver, nickel, gold or aluminum.
[0054] FIG. 11A shows a cross section of an LED lamp 1100 according to
another embodiment of the invention. A substrate 1106 is a "gull-wing"
substrate. A "J-lead" substrate is similar to the gull-wing substrate in
that it provides external leads for surface mounting of the LED lamp. The
gull-wing substrate is a metal leadframe formed in a gull-wing SMT
configuration. The metal leadframe may include a reflector cup 1110
formed on it and plated with a reflective surface. The substrate 106 may
be configured to have 2 or more leads 1112, 1114 for its electrical
termination, and not limited to the example shown in FIG. 11A. In one
aspect of implementation of the invention, the plating material forming
the reflective surface of the reflector cup in the metal leadframe or the
PCB may include, but not limited to, silver, nickel or gold. FIG. 11B is
a plan view of the substrate of FIG. 11A.
[0055] FIGS. 12-14 are isometric views of receptacles 1200, 1300, 1400
configured to accommodate more than one LED chip. Receptacle 1200 has
three mold cups 1202, 1204, 1206 and a perimeter rim 1208 surrounding the
three mold cups 1202, 1204, 1206. Elastomeric encapsulant is dispensed to
fill the receptacle to a desired height. LEDs mounted on a substrate or
substrates are inserted into the elastomeric encapsulant. In a particular
embodiment, three LEDs are mounted on a single, rectangular substrate,
which is inserted into the receptacle so that the LEDs transmit light
into the mold cups, and are surrounded by elastomeric encapsulant. The
rim 1208 aligns the substrate in the receptacle, which positions the LEDs
in the mold cups.
[0056] FIG. 13 shows a receptacle 1300 having a single mold cup/lens 1302
into which elastomeric encapsulant is dispensed. A plurality of LED chips
on a plurality of substrates, or a plurality of LED chips on a single
substrate, are then inserted into the elastomeric encapsulant through the
oval aperture 1304. FIG. 14 shows another receptacle 1400 having six mold
cups/lenses and a perimeter rim 1402.
[0057] One or more LED chips may be attached to a substrate, and a
substrate may have two or more electrical terminations. The shape of a
receptacle and the number of lens outline may be configured to match the
number of LEDs and the outline of the substrate on which the LEDs are
disposed. The configurations shown in figures are representative of the
shapes of transparent receptacles used, and the invention is not limited
to these shapes.
[0058] FIG. 15 is a flow chart of a method of manufacturing an LED device
1500 according to an embodiment of the invention. An LED chip is
die-attached to a substrate (step 1502) and is electrically connected to
leads (step 1504). In one embodiment, the substrate is connected to a
first terminal of the LED chip with a conductive die-attach technique,
and is connected to a second terminal of the LED chip with a wire bond.
In an alternative embodiment, a first wire bond couples a first terminal
of the LED chip to a first lead (e.g. a substrate integrated with the
first lead), and a second wire bond connects a second terminal of the LED
chip to a second lead. In a particular embodiment, the first and second
leads extend from a leadframe strip. In an alternative embodiment, the
substrate is a PCB substrate or an array of PCB substrates that, when
singulated, form individual LED devices. In a particular embodiment, a
wavelength-converting layer of phosphor particles dispersed in a matrix
(e.g. silicone, other elastomeric material, or sol gel) is disposed on
the LED chip.
[0059] Uncured elastomeric encapsulant is dispensed into a receptacle
(step 1506), and the LED chip and substrate are inserted into the uncured
elastomeric encapsulant (step 1508). The elastomeric encapsulant is then
cured (step 1510). In a particular embodiment, the entire LED device or
LED leadframe strip, including any alignment fixtures, is cured in a
conveyor-belt oven or alternatively a box oven.
[0060] The receptacle is made of a material, typically plastic, that is
harder and more rigid than the cured elastomeric encapsulant material. In
one embodiment, the LED chip is inserted into the uncured elastomeric
encapsulant so as to surround the LED chip with uncured elastomeric
encapsulant. Generally, the volume of the receptacle is known, and a
selected amount of encapsulant is dispensed into the receptacle. The
LED-substrate subassembly is inserted into the uncured elastomeric
encapsulant to a selected depth so that the uncured elastomeric
encapsulant fills the receptacle to a desired level.
[0061] In a particular embodiment, the LED chip, bond wire(s), substrate,
and a portion of the leads are inserted into the uncured elastomeric
encapsulant. In a particular embodiment, the LED chip lies within a
reflector cup, and the method includes an optional step of pre-filling
the reflector cup with uncured elastomeric encapsulant before the LED
chip and substrate are inserted into the elastomeric encapsulant
dispensed into the receptacle. Pre-filling the reflector cup avoids
trapping gas that can form a bubble(s) near the LED chip, which can
interfere with light emission from the LED device and thermal sinking of
the LED chip.
[0062] In a further embodiment, a second encapsulant is dispensed into the
receptacle over the elastomeric encapsulant (step 1512) and then cured
(step 1514). In a particular embodiment, the cured second encapsulant is
more rigid than the elastomeric encapsulant and surrounds and secures the
leads. In some embodiments, the second encapsulant provides a second seal
across the aperture that is stronger and more durable than a first seal
provided by the elastomeric encapsulant. In a yet further embodiment, the
leads are cut to singulate the LED device from a leadframe strip (step
1516), or alternatively, an LED device having a PCB substrate is
singulated from an array of PCB substrates.
[0063] The combination of a hard outer receptacle and inner elastomeric
material provides several advantages over conventional LED lamps.
Referring to FIG. 1A, if an epoxy is used for the encapsulant 116,
stresses resulting from curing can damage the wire bond. Thermal stress
also arises from differences in the thermal coefficient of expansion
between the epoxy and the substrate during thermal shock, such as during
a solder re-flow process. Excessive stress can lift or crack the LED chip
as the lamp is heated or cooled. The elastomeric encapsulant provides
lower stress in the package and better reliability and yields after
thermal shock, such as solder re-flow. Similarly, the elastomeric
encapsulant avoids degradation of light output (yellowing), particularly
with medium- to low-powered LED chips (i.e. less than 500 mW) emitting
light having short wavelength (i.e. less than 570 nm) LED chips. The
receptacle provides rigidity and wear resistance for the elastomeric
encapsulant. However, embodiments are also suitable for use with LED
devices emitting wavelengths greater than 570 nm at power greater than
500 mW. Embodiments of the invention are easily adaptable to leadframe
LED manufacturing techniques, providing simple construction with few
piece-parts and ease of manufacturing in high volume production
[0064] While the preferred embodiments of the present invention have been
illustrated in detail, it should be apparent that modifications and
adaptations to these embodiments might occur to one skilled in the art
without departing from the scope of the present invention as set forth in
the following claims.
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